The present invention relates to an ink jet head and an ink jet type recording apparatus.
In recent years, there has been proposed an ink jet type recording apparatus which discharges, during one printing cycle for forming a single dot on recording paper, a plurality of ink droplets from the same nozzle of an ink jet head so as to form a single dot by the plurality of ink droplets, as disclosed in, for example, Japanese Laid-Open Patent Publication No. 10-81012.
An ink jet type recording apparatus of this type includes an ink jet head for discharging ink droplets and relative movement means for relatively moving the ink jet head and the recording paper with respect to each other. The ink jet head includes a head body which is provided with a pressure chamber containing ink and a nozzle, an actuator for discharging the ink in the pressure chamber through the nozzle, and driving signal supply means for supplying driving signals to the actuator.
While the ink jet head and the recording paper are relatively moved with respect to each other by the relative movement means, the driving signal supply means supplies a driving signal including one or more driving pulses during one printing cycle. The actuator is actuated by receiving the driving signal so as to discharge one or more ink droplets through the nozzle. The ink droplets thus discharged strike the recording paper in the order they are discharged so as to form a single ink dot. A predetermined image is formed on the recording paper by a collection of a large number of such ink dots on the recording paper. In this process, the number of ink droplets to be discharged during one printing cycle is adjusted so as to adjust the gradation and the size of the dot, thereby realizing so-called xe2x80x9cmultiple gray level printingxe2x80x9d.
In order to discharge one or more ink droplets during one printing cycle as described above, it is necessary to supply to the actuator a number of driving pulses corresponding to the number of ink droplets to be discharged. However, it is difficult to form a desirable ink dot on the recording paper simply by supplying a number of driving pulses corresponding to the number of ink droplets to be discharged without elaborating the driving signal.
For example, when performing high-speed printing, the speed of the relative movement of the ink jet head and the recording paper is high, whereby it is likely that a plurality of ink droplets discharged from the same nozzle strike the recording paper at positions shifted from each other. As a result, it is likely that the ink dot has an oblong circle shape, thereby deteriorating the printing quality. Therefore, in such a case, it is necessary to successively discharge a plurality of ink droplets so that the ink droplet discharging interval is reduced as much as possible, and to discharge the ink droplets so that each later discharged ink droplet is discharged with a higher discharge velocity than that of the previously discharged ink droplet. Thus, a new technique for precisely discharging ink droplets successively with higher velocities in the order they are discharged has been longed for.
On the other hand, there has also been proposed a method in which two ink droplets discharged from the same nozzle are allowed to merge in flight into a single ink droplet before striking, as disclosed in, for example, U.S. Pat. No. 5,285,215 or Japanese Patent Publication for Opposition No. 7-108568. In such a method, it is necessary to elaborate, particularly, the driving signal. In the apparatus disclosed in Japanese Patent Publication for Opposition No. 7-108568, it is made possible to change the discharge velocity of an ink droplet by changing the inclination angle of the trailing edge portion of a driving pulse.
However, supplying a driving signal including a plurality of driving pulses whose trailing edge portions have different inclination angles to the actuator has complicated the driving signal supply means and increased the cost thereof. In such a background, a new technique for merging a plurality of ink droplets before striking using a driving signal of a simple waveform has been longed for.
Moreover, if the meniscus vibration of ink upon completion of one printing cycle remains in the next printing cycle, the ink discharging performance will be unstable. In view of this, a method for supplying a driving signal which is less susceptible to the adverse influence of the meniscus vibration has been longed for.
The present invention has been made in view of the above, and has an object to improve the ink discharging performance of an ink jet head which discharges one or more ink droplets from the same nozzle during one printing cycle, and the ink discharging performance of an ink jet type recording apparatus incorporating the same.
An ink jet head according to the present invention includes: a head body which is provided with a pressure chamber containing ink and a nozzle communicated to the pressure chamber; an actuator having a piezoelectric element for applying a pressure on the ink in the pressure chamber by a piezoelectric effect of the piezoelectric element; and driving signal supply means for supplying a driving voltage signal including a plurality of driving pulses to the piezoelectric element of the actuator, wherein the driving signal supply means supplies the plurality of driving pulses during one predetermined printing cycle so that a time interval between the driving pulses gradually approaches a natural period of the actuator.
Note that the natural period of the actuator as used herein refers to the natural period of the entire vibration system including an acoustic element (specifically, the ink).
Thus, a plurality of driving pulses are supplied to the piezoelectric element of the actuator during one printing cycle, thereby discharging a plurality of ink droplets from the same nozzle. Now, the time interval of the plurality of driving pulses gradually approaches the natural period of the actuator, whereby the discharge velocity of the plurality of ink droplets discharged from the nozzle gradually increases. Therefore, a later discharged ink droplet has a higher discharge velocity than that of a previously discharged ink droplet. Then, the later discharged ink droplet catches up with the previously discharged ink droplet, and the ink droplets merge before striking the recording medium. As a result, the plurality of ink droplets merge into a single ink droplet and then strike the recording medium, thereby forming a desirable single dot on the recording medium.
It is preferred that the driving signal supply means supplies the plurality of driving pulses so that the time interval between the driving pulses gradually increases.
Thus, the time interval of the driving pulse gradually increases so as to approach the natural period of the actuator, whereby the overall time interval of the driving pulses is shorter than that when the time interval gradually decreases so as to approach the natural period. Therefore, it is possible to reduce the printing cycle, thereby enabling printing at a higher speed.
In the ink jet head described above, the driving voltage signal may include a negative pressure potential for driving the actuator to depressurize the pressure chamber, and a positive pressure potential for driving the actuator to pressurize the pressure chamber; and the plurality of driving pulses may include: an initial driving pulse composed of a potential decreasing waveform which decreases from a predetermined reference potential between a negative pressure potential and a positive pressure potential to the negative pressure potential, a negative pressure potential holding waveform which holds the negative pressure potential, and a potential increasing waveform which increases from the negative pressure potential to the positive pressure potential; and one or more subsequent driving pulses each composed of a positive pressure potential holding waveform which holds a positive pressure potential, a potential decreasing waveform which decreases from the positive pressure potential to a negative pressure potential, a negative pressure potential holding waveform which holds the negative pressure potential, and a potential increasing waveform which increases from the negative pressure potential to a positive pressure potential.
Thus, so-called pull-push type ink discharge is performed in which the actuator is once driven to be depressurized and then driven to be pressurized so as to discharge the ink.
The driving signal supply means may be configured so as to sequentially supply at least the initial driving pulse, a first subsequent driving pulse and a second subsequent driving pulse during one printing cycle; and a first time t1 from a start of potential decrease in the potential decreasing waveform to an end of potential increase in the potential increasing waveform in the initial driving pulse, a second time t2 from a start of potential holding in a positive pressure potential holding waveform to an end of potential increase in a potential increasing waveform in the first subsequent driving pulse, and a third time t3 from a start of potential holding in a positive pressure potential holding waveform to an end of potential increase in a potential increasing waveform in the second subsequent driving pulse, may be set to satisfy t1xe2x89xa6t2 less than t3xe2x89xa6t0 with respect to the natural period t0 of the actuator.
Thus, the first ink droplet discharged by the initial driving pulse, the second ink droplet discharged by the first subsequent driving pulse, and the third ink droplet discharged by the second subsequent driving pulse, merge before striking the recording medium, thereby forming a single dot on the recording medium. As a result, a desirable single dot is formed on the recording medium, and high-speed printing is enabled.
The positive pressure potential of the initial driving pulse and the positive pressure potential of each of the subsequent driving pulses may be equal to each other; and the negative pressure potential of the initial driving pulse and the negative pressure potential of each of the subsequent driving pulses may be equal to each other.
Thus, a plurality of driving pulses are formed by three levels of potential, i.e., the predetermined positive pressure potential, the predetermined reference potential, and the predetermined negative pressure potential. Therefore, the driving pulses can be easily formed.
It is preferred that a time T1 from a start of potential decrease in the potential decreasing waveform of the initial driving pulse to an end of potential increase in a potential increasing waveform of a last subsequent driving pulse in one printing cycle is set to satisfy T1/T2xe2x89xa60.5 with respect to a minimum printing cycle T2.
Thus, a sufficient time for settling down the ink in the pressure chamber is ensured between when the last subsequent driving pulse is supplied and when the initial driving pulse of the next printing cycle is supplied. Therefore, the ink discharge is stabilized.
With an actuator whose natural period is relatively long, the influence of the waveform holding time of the potential holding waveform of a driving pulse on the ink discharge velocity is relatively small. Therefore, by shortening the potential holding waveform, the potential increasing waveform or the potential decreasing waveform can be elongated accordingly.
In view of this, it is preferred that a pulse width of each of the driving pulses is set to be less than or equal to the natural period of the actuator; and a waveform holding time of a potential holding waveform of each of the driving pulses is set to be less than or equal to xc2xc of the natural period of the actuator.
Thus, the rising time of the potential increasing waveform or the falling time of the potential decreasing waveform is sufficiently ensured, thereby realizing stable ink discharge without extra dots. Note that the waveform holding time may be zero. In other words, the waveform holding time may be 0 to xc2xc times the natural period.
Alternatively, in the ink jet head described above, the plurality of driving pulses may include three or more rectangular driving pulses each composed of a potential increasing waveform which increases from a predetermined reference potential to a positive pressure potential for driving the actuator to pressurize the pressure chamber, a positive pressure potential holding waveform which holds the positive pressure potential, and a potential decreasing waveform which decreases from the positive pressure potential to a predetermined reference potential.
Thus, three or more rectangular driving pulses are supplied to the actuator during one printing cycle so that three or more ink droplets are discharged from the nozzle so that the discharge velocity gradually increases. As a result, the ink droplets merge before striking the recording medium, so that they strike the recording medium after merging into a single ink droplet. Therefore, a desirable single dot is formed on the recording medium, and high-speed printing is enabled.
The driving signal supply means may be configured so as to sequentially supply at least first, second and third rectangular driving pulses during one printing cycle; and a first time t1 from an end potential increase in the first driving pulse to an end of potential increase in the second driving pulse, and a second time t2 from an end of potential increase in the second driving pulse to an end of potential increase in the third driving pulse, may be set to satisfy t1 less than t2xe2x89xa6t0 with respect to the natural period t0 of the actuator.
Thus, the first ink droplet discharged by the first driving pulse, the second ink droplet discharged by the second driving pulse, and the third ink droplet discharged by the third driving pulse, merge before striking the recording medium, thereby forming a single dot on the recording medium.
The rectangular driving pulses may have an equal positive pressure potential and an equal reference potential.
Thus, the driving pulses can be formed by only two potentials, whereby the driving pulses can be formed easily.
It is preferred that a time T1 from a start of potential increase in a first driving pulse to a start of potential increase in a last driving pulse in one printing cycle is set to satisfy T1/T2xe2x89xa60.5 with respect to a minimum printing cycle T2.
Thus, a sufficient time for settling down the ink in the pressure chamber is ensured between when the last driving pulse is supplied and when the first driving pulse of the next printing cycle is supplied. Therefore, the ink discharge is stabilized.
Another ink jet head according to the present invention includes a head body which is provided with a pressure chamber containing ink and a nozzle communicated to the pressure chamber; an actuator having a piezoelectric element for applying a pressure on the ink in the pressure chamber by a piezoelectric effect of the piezoelectric element; and driving signal supply means for supplying a driving voltage signal to the piezoelectric element of the actuator, wherein: the driving signal supply means is configured so as to supply a plurality of driving pulses during one predetermined printing cycle; and a time interval between the driving pulses increases so as to gradually approach a predetermined time which is slightly longer than a natural period of the actuator so that a later discharged ink droplet has a higher discharge velocity than that of a previously discharged ink droplet.
When a plurality of driving pulses are successively supplied within a short period of time, the influence of the vibration of the actuator or the pulsation of the ink from a preceding driving pulse may remain, thereby influencing the actuation of the actuator by a subsequent driving pulse. As a result, there are cases where the ink discharge velocity is higher if the time interval between driving pulses is set to be equal to a predetermined time which is slightly longer than the natural period of the actuator than that obtained if it is set to be equal to the natural period. Therefore, in such cases, the discharge velocity of the ink droplets may be increased in the order they are discharged so that the ink droplets merge before striking by gradually increasing the time interval between driving pulses so that the time interval approaches a predetermined time which is slightly longer than the natural period as described above.
Another ink jet head according to the present invention includes a head body which is provided with a pressure chamber containing ink and a nozzle communicated to the pressure chamber; an actuator having a piezoelectric element for applying a pressure on the ink in the pressure chamber by a piezoelectric effect of the piezoelectric element; and driving signal supply means for supplying a driving voltage signal to the piezoelectric element of the actuator, wherein: the driving signal supply means is configured so as to supply a plurality of driving pulses during one predetermined printing cycle; and the plurality of driving pulses are supplied in such an order that a pulse width thereof gradually approaches a time which is equal to, or approximately equal to, one half of a natural period of the actuator so that a later discharged ink droplet has a higher discharge velocity than that of a previously discharged ink droplet.
Thus, a plurality of driving pulses are supplied to the piezoelectric element of the actuator during one printing cycle, thereby discharging a plurality of ink droplets from the same nozzle. Now, since the pulse width of the driving pulse gradually approaches the time which is equal to, or approximately equal to, one half of the natural period of the actuator, the discharge velocity of the ink droplets discharged from the nozzle gradually increases in the order they are discharged. Therefore, a later discharged ink droplet catches up with a previously discharged ink droplet, whereby the ink droplets merge before striking the recording medium. As a result, the plurality of ink droplets merge into a single ink droplet and then strike the recording medium, thereby forming a desirable single dot on the recording medium.
The driving voltage signal may include a negative pressure potential for driving the actuator to depressurize the pressure chamber, and a positive pressure potential for driving the actuator to pressurize the pressure chamber; and the plurality of driving pulses may include: an initial driving pulse composed of a potential decreasing waveform which decreases from a predetermined reference potential between a negative pressure potential and a positive pressure potential to the negative pressure potential, a negative pressure potential holding waveform which holds the negative pressure potential, and a potential increasing waveform which increases from the negative pressure potential to the positive pressure potential; and one or more subsequent driving pulses each composed of a positive pressure potential holding waveform which holds a positive pressure potential, a potential decreasing waveform which decreases from the positive pressure potential to a negative pressure potential, a negative pressure potential holding waveform which holds the negative pressure potential, and a potential increasing waveform which increases from the negative pressure potential to a positive pressure potential.
Alternatively, the driving voltage signal may include a predetermined reference potential, and a negative pressure potential for driving the actuator to depressurize the pressure chamber; and the plurality of driving pulses may include three or more driving pulses each composed of a potential decreasing waveform which decreases from a reference potential to a negative pressure potential, a negative pressure potential holding waveform which holds the negative pressure potential, and a potential increasing waveform which increases from the negative pressure potential to a reference potential.
Thus, so-called pull-push (pull and push) type ink discharge is performed, whereby a plurality of ink droplets are discharged during one printing cycle. Since the pulse width of the driving pulse gradually approaches the time which is equal to, or approximately equal to, one half of the natural period of the actuator, the plurality of ink droplets merge before striking, so that they strike the recording medium after merging into a single ink droplet.
Moreover, the plurality of driving pulses may include three or more rectangular driving pulses each composed of a potential increasing waveform which increases from a predetermined reference potential to a positive pressure potential for driving the actuator to pressurize the pressure chamber, a positive pressure potential holding waveform which holds the positive pressure potential, and a potential decreasing waveform which decreases from the positive pressure potential to the reference potential.
Thus, three or more rectangular driving pulses are supplied to the actuator during one printing cycle, whereby three or more ink droplets are discharged from the nozzle so that the discharge velocity thereof gradually increases. As a result, the ink droplets merge before striking the recording medium, so that they strike the recording medium after merging into a single ink droplet.
It is preferred that the plurality of driving pulses are supplied in such an order that a pulse width thereof gradually increases.
Thus, the pulse width of the driving pulse gradually increases so as to approach the time which is equal to, or approximately equal to, one half of the natural period, whereby the time obtained as the total pulse width of the driving pulses is shorter than that when the pulse width gradually decreases so as to approach the time. Therefore, the printing cycle can be reduced, and printing at a higher speed is enabled.
Another ink jet head according to the present invention includes: a head body which is provided with a plurality of pressure chambers containing ink and a plurality of nozzles communicated to the pressure chambers, respectively; a plurality of actuators each having a piezoelectric element for applying a pressure on the ink in the respective pressure chambers by a piezoelectric effect of the piezoelectric element; a driving signal production section for producing a reference driving signal including, in one predetermined printing cycle, N (N is a natural number equal to or greater than 2) ink discharging pulse signals for driving the actuators so as to discharge ink droplets from the nozzles; and a signal selection section for selectively supplying, to one of the actuators, P (P is a natural number less than or equal to N) ink discharging pulse signals included in the reference driving signal, wherein: the ink discharging pulse signals of the reference driving signal are formed so that a later discharged ink droplet has a higher discharge velocity than that of a previously discharged ink droplet; and the signal selection section is configured so as to supply Nxe2x88x92P+1th and subsequent ink discharging pulse signals of the reference driving signal.
Thus, the driving signal production section produces a reference driving signal including N ink discharging pulse signals so that a maximum of N ink droplets can be discharged during one printing cycle. On the other hand, the signal selection section selects, and supplies to the actuator, a total of P ink discharging pulse signals, i.e., the Nxe2x88x92P+1th and subsequent signals, of the N ink discharging pulse signals so as to discharge P ink droplets during one printing cycle, according to a predetermined image signal. Now, since the P ink discharging pulse signals are pulse signals which have been successively produced in the reference driving signal, the time interval between the pulses is short. Therefore, the P ink droplets will be successively discharged one right after another. Moreover, the N ink discharging pulse signals are formed so that a later discharged ink droplet has a higher discharge velocity than that of a previously discharged ink droplet, whereby the total of P ink droplets, which are discharged by the P ink discharging pulse signals, are discharged so that the discharge velocity thereof successively increases. Therefore, there is only a little displacement among the positions at which the P ink droplets strike, and merging the P ink droplets before striking is facilitated. Thus, irrespective of the number of ink droplets to be discharged, a desirable ink dot is formed and the ink discharging performance is improved.
Moreover, the driving signal produced by the driving signal production section for discharging the ink is only one kind of reference driving signal, and it is therefore not necessary to separately produce a number of driving signals according to the number of ink discharges. Thus, the configuration of the control system is simplified, and the cost is reduced.
It is preferred that the driving signal production section produces an auxiliary pulse signal for suppressing meniscus vibration of the ink in the head body after producing the reference driving signal; and the signal selection section is configured so as to supply, to one of the actuators, the Nxe2x88x92P+1th and subsequent ink discharging pulse signals and the auxiliary pulse signal.
Thus, an auxiliary pulse signal is supplied to the actuator after a total of P ink discharging pulse signals, i.e., the Nxe2x88x92P+1th and subsequent signals, of the reference driving signal are supplied thereto. As a result, the meniscus vibration of the ink after discharging the P ink droplets is suppressed, and the ink discharging performance in the next printing cycle is stabilized.
Another ink jet head according to the present invention includes: a head body which is provided with a plurality of pressure chambers containing ink and a plurality of nozzles communicated to the pressure chambers, respectively; a plurality of actuators each having a piezoelectric element for applying a pressure on the ink in the respective pressure chambers by a piezoelectric effect of the piezoelectric element; a driving signal production section for producing a reference driving signal including, in one predetermined printing cycle, N (N is a natural number equal to or greater than 2) ink discharging pulse signals for driving the actuators so as to discharge ink droplets from the nozzles; and a signal selection section for selectively supplying, to one of the actuators, P (P is a natural number less than or equal to N) ink discharging pulse signals included in the reference driving signal, wherein: the ink discharging pulse signals of the reference driving signal are formed so that a later discharged ink droplet has a higher discharge velocity than that of a previously discharged ink droplet; the driving signal production section produces an auxiliary pulse signal for suppressing meniscus vibration of the ink in the head body after producing the reference driving signal; and the signal selection section is configured so as to supply the first ink discharging pulse signal of the reference driving signal when P is 1, and to supply the Nxe2x88x92P+1th and subsequent ink discharging pulse signals of the reference driving signal and the auxiliary pulse signal when P is equal to or greater than 2.
Thus, in a case where one (P=1) ink droplet is discharged during one printing cycle, only the first pulse signal of the N ink discharging pulse signals included in the reference driving signal is supplied to the actuator. The first pulse signal has a stable waveform as compared to those of the second and subsequent pulse signals, and it is produced in the earliest period in the printing cycle, whereby the ink discharging timing is precise while the ink discharging performance is stabilized and the precision of the position at which the ink strikes is improved. Note that in such a case, since only one ink droplet is discharged, the overall amount of ink discharged during one printing cycle is small, and the influence of the meniscus vibration is small. Therefore, there is no problem even if the auxiliary pulse signal is not supplied. Moreover, even in a case where two or more (Pxe2x89xa72) ink droplets are discharged during one printing cycle, a desirable ink dot is formed irrespective of the number of ink droplets to be discharged, and the ink discharging performance is improved, for the reasons described above. Note that in such a case, an auxiliary pulse signal is supplied to the actuator after the reference driving signal is supplied thereto, thereby suppressing the deterioration in the discharging performance due to the influence of the meniscus vibration.
It is preferred that an interval between the Nth ink discharging pulse signal of the reference driving signal and the auxiliary pulse signal is set to be 0.5 to 1.5 times a natural period of the actuators. Note that the natural period of the actuator refers to the natural period of the entire vibration system including an acoustic element (specifically, the ink).
Thus, the meniscus vibration of the ink is efficiently suppressed.
While it is difficult to sufficiently suppress the meniscus vibration when the potential difference of the auxiliary pulse signal is too small, an unintended ink discharge may occur when the potential difference is too large. In view of this, it is preferred that a potential difference of the auxiliary pulse signal is set to be 0.1 to 0.3 times a minimum potential difference of the ink discharging pulse signals of the reference driving signal.
Thus, there is obtained an auxiliary pulse signal which is suitable for efficiently suppressing the meniscus vibration without discharging the ink.
Each ink discharging pulse signal of the reference driving signal may be composed of a rectangular or trapezoidal pulse signal having a first potential as a reference potential and a second potential which is different from the first potential; the signal selection section may be comprised of a switching circuit which is selectively switched to either one of an ON state where the reference driving signal is supplied to one of the actuators and an OFF state where the supply of the reference driving signal to the actuator is stopped; and the switching circuit may be configured so as to be switched from the OFF state to the ON state while a potential of the reference driving signal is at the first potential.
Thus, the ink discharging pulse signals of the reference driving signal are composed of rectangular or trapezoidal pulse signals with only two potentials, i.e., the first potential and the second potential, whereby the waveform of the reference driving signal is simplified. Therefore, the configuration of the driving signal production section for producing the reference driving signal is simplified.
The ink discharging pulse signals of the reference driving signal may be each composed of: an initial pulse signal composed of a potential decreasing waveform which decreases from a reference potential, which is between a negative pressure potential for driving one of the actuators to depressurize one of the pressure chambers and a positive pressure potential for driving the actuator to pressurize the pressure chamber, to the negative pressure potential, a negative pressure potential holding waveform which holds the negative pressure potential, and a potential increasing waveform which increases from the negative pressure potential to the positive pressure potential; and one or more subsequent pulse signals each composed of a potential decreasing waveform which decreases from a respective one of predetermined positive pressure potentials to a respective one of predetermined negative pressure potentials, a negative pressure potential holding waveform which holds the respective one of the negative pressure potentials, and a potential increasing waveform which increases from the respective one of the negative pressure potentials to a respective one of predetermined positive pressure potentials; the signal selection section may be comprised of a switching circuit which is selectively switched to either one of an ON state where the reference driving signal is supplied to one of the actuators and an OFF state where the supply of the reference driving signal to the actuator is stopped; and the switching circuit may be configured so as to be switched from the OFF state to the ON state after passage of a predetermined time from a start of waveform holding in the negative pressure potential holding waveform of the reference driving signal so that the supply of the reference driving signal is started after a potential of the reference driving signal has transitioned to the negative pressure potential.
Thus, the switching circuit is switched from the OFF state to the ON state after passage of a predetermined time from the start of waveform holding of the negative pressure potential holding waveform of the reference driving signal so that it is switched with a predetermined time delay from the falling transition of the waveform of the reference driving signal. Therefore, the switching circuit is not switched while the potential of the reference driving signal is decreasing, whereby no unstable driving signal including a holding waveform of an unintended potential other than the reference potential, the negative pressure potential and the positive pressure potential is supplied to the actuator.
Each ink discharging pulse signal of the reference driving signal may be composed of a potential decreasing waveform which decreases from a reference potential to a negative pressure potential for driving one of the actuators to depressurize one of the pressure chambers, a negative pressure potential holding waveform which holds the negative pressure potential, and a potential increasing waveform which increases from the negative pressure potential to the reference potential; the auxiliary pulse signal may be composed of a potential decreasing waveform which decreases from the reference potential to an auxiliary negative pressure potential for driving one of the actuators to depressurize one of the pressure chambers, a negative pressure potential holding waveform which holds the auxiliary negative pressure potential, and a potential increasing waveform which increases from the auxiliary negative pressure potential to the reference potential; and an interval between an end of potential increase in the potential increasing waveform in the Nth ink discharging pulse signal of the reference driving signal and a start of potential decrease in the potential decreasing waveform of the auxiliary pulse signal may be set to be 0.5 to 1 times a natural period of the actuators.
Thus, it is possible to realize stable ink discharge by using a reference driving signal having potential holding waveforms for two potentials (the reference potential and the negative pressure potential) and utilizing the ink discharging function and the meniscus vibration suppressing function based on the so-called pull-push operation of the actuator.
The ink discharging pulse signals of the reference driving signal may be each composed of: an initial pulse signal composed of a potential decreasing waveform which decreases from a reference potential, which is between a negative pressure potential for driving one of the actuators to depressurize one of the pressure chambers and a positive pressure potential for driving the actuator to pressurize the pressure chamber, to the negative pressure potential, a negative pressure potential holding waveform which holds the negative pressure potential, and a potential increasing waveform which increases from the negative pressure potential to the positive pressure potential; and one or more subsequent pulse signals each composed of a potential decreasing waveform which decreases from a respective one of predetermined positive pressure potentials to a respective one of predetermined negative pressure potentials, a negative pressure potential holding waveform which holds the respective one of the negative pressure potentials, and a potential increasing waveform which increases from the respective one of the negative pressure potentials to a respective one of predetermined positive pressure potentials; the auxiliary pulse signal may be composed of a potential decreasing waveform which decreases from the reference potential to an auxiliary negative pressure potential for driving one of the actuators to depressurize one of the pressure chambers, a negative pressure potential holding waveform which holds the auxiliary negative pressure potential, and a potential increasing waveform which increases from the auxiliary negative pressure potential to the reference potential; and an interval between an end of potential increase in the potential increasing waveform in the last subsequent pulse signal of the reference driving signal and a start of potential decrease in the potential decreasing waveform of the auxiliary pulse signal may be set to be 0.5 to 1 times a natural period of the actuators.
Thus, it is possible to realize stable ink discharge by using a reference driving signal having potential holding waveforms for three potentials (the reference potential, the negative pressure potential and the positive pressure potential) and utilizing the ink discharging function and the meniscus vibration suppressing function based on the so-called pull-push operation of the actuator.
The ink discharging pulse signals of the reference driving signal may be each composed of: an initial pulse signal composed of a potential decreasing waveform which decreases from a reference potential, which is between a negative pressure potential for driving one of the actuators to depressurize one of the pressure chambers and a positive pressure potential for driving the actuator to pressurize the pressure chamber, to the negative pressure potential, a negative pressure potential holding waveform which holds the negative pressure potential, and a potential increasing waveform which increases from the negative pressure potential to the positive pressure potential; and one or more subsequent pulse signals each composed of a potential decreasing waveform which decreases from a respective one of predetermined positive pressure potentials to a respective one of predetermined negative pressure potentials, a negative pressure potential holding waveform which holds the respective one of the negative pressure potentials, and a potential increasing waveform which increases from the respective one of the negative pressure potentials to a respective one of predetermined positive pressure potentials; the auxiliary pulse signal may be composed of a potential increasing waveform which increases from the reference potential to an auxiliary pressurizing potential for driving one of the actuators to pressurize one of the pressure chambers, a positive pressure potential holding waveform which holds the auxiliary positive pressure potential, and a potential decreasing waveform which decreases from the auxiliary positive pressure potential to the reference potential; and an interval between an end of potential increase in the potential increasing waveform in the last subsequent pulse signal of the reference driving signal and a start of potential increase in the potential increasing waveform of the auxiliary pulse signal may be set to be 1 to 1.5 times a natural period of the actuators.
Thus, it is possible to realize stable ink discharge by using a reference driving signal having potential holding waveforms for three potentials (the reference potential, the negative pressure potential and the positive pressure potential) and utilizing the ink discharging function based on the so-called pull-push operation of the actuator and the meniscus vibration suppressing function based on the so-called push-pull operation.
The ink discharge velocity is higher as the interval between the ink discharging pulse signals supplied to the actuator is closer to the natural period of the actuator. In view of this, the reference driving signal may be formed so that an interval between the N ink discharging pulse signals gradually approaches a natural period of the actuators while gradually increasing.
Thus, a later discharged ink droplet has a higher discharge velocity than that of a previously discharged ink droplet, thereby obtaining a specific preferable reference driving signal.
On the other hand, the ink discharge velocity is higher as the pulse height (potential difference) of the ink discharging pulse signal supplied to the actuator is greater. In view of this, the reference driving signal may be formed so that a potential differences in the N ink discharging pulse signals gradually increase.
Thus, a later discharged ink droplet has a higher discharge velocity than that of a previously discharged ink droplet, thereby obtaining a specific preferable reference driving signal.
The thickness of the piezoelectric element may be set to be 0.5 xcexcm to 5 xcexcm. Also when the piezoelectric element is thus provided as a thin film, a desirable dot is formed on the recording medium.
An ink jet type recording apparatus according to the present invention includes: the ink jet head as described above; and relative movement means for relatively moving the ink jet head and a recording medium with respect to each other while the ink jet head discharges ink.
Thus, it is possible to obtain an ink jet type recording apparatus with an excellent ink discharging performance.
As described above, according to the present invention, a plurality of driving pulses are supplied to the actuator, and the time interval between the pulses is set so as to gradually approach the natural period of the actuator or a predetermined time which is slightly longer than the natural period, whereby it is possible to discharge a plurality of ink droplets so that the discharge velocity thereof gradually increases. Thus, it is possible to merge a plurality of ink droplets before striking the recording medium, so that they strike the recording medium as a single ink droplet. Therefore, a desirable single dot can be formed on the recording medium from a plurality of ink droplets. As a result, the printing quality and/or the printing speed can be improved.
In such a case, by gradually increasing the time interval of the driving pulse, it is possible to reduce the time of one printing cycle and to increase the speed of printing.
Moreover, according to the present invention, a plurality of driving pulses are supplied to the actuator, and the pulse width thereof is set so as to gradually approach a time which is equal to, or approximately equal to, one half of the natural period of the actuator, whereby it is possible to discharge a plurality of ink droplets so that the discharge velocity thereof gradually increases. Thus, it is possible to merge a plurality of ink droplets so that they strike the recording medium as a single ink droplet, and to improve the printing quality and/or the printing speed.
In such a case, by gradually increasing the pulse width of the driving pulse, it is possible to reduce the time of one printing cycle and to increase the speed of printing.
Moreover, according to the present invention, in a case where P ink droplets are discharged during one printing cycle, a reference driving signal including N pulse signals is produced in the driving signal production section, and the Nxe2x88x92P+1th and subsequent pulse signals of the reference driving signal are supplied to the actuator, whereby a desirable ink dot can be formed on the recording medium from a plurality of ink droplets.
Moreover, according to the present invention, the first pulse signal of the reference driving signal is supplied when P is 1, and the Nxe2x88x92P+1th and subsequent pulse signals of the reference driving signal and an auxiliary pulse signal are supplied when P is equal to or greater than 2, whereby the discharging performance for a case where one ink droplet is discharged can be further improved.